1. Field of the Invention
The present invention relates to an electrostatographic printing machine of the duplex copying type, and more particularly to an electrostatographic printing machine which forms toner copies of images on both sides of a print sheet of paper, and fuses and permanently fixes the toner copies onto both sides of the print sheet by a fusing unit, a roll biased for paper transportation being provided between the transfer unit and the fusing unit of the printing machine.
2. Description of the Related Art
In the field of printers, copying machines and the like, the need of the duplexing printing in which images, for example, are printed on both sides of a print substrate is increasing on social and market demands for saving resources and reducing the cost of transporting printed matters. The continuous paper printer employs a called tandem printing system. In the tandem printing system, two laser printers are used. A first laser printer prints a toner image on one side of the print substrate and a second one does the other side of the substrate. Recently, an attraction has paid to a printing system in which unfixed toner powder images are formed on both sides, and those toner images are simultaneously fused and fixed on both sides of the print substrate. A construction of this simultaneous duplexing printing type is schematically illustrated in FIG. 6. Description will be given using a printing unit which uses a positively charged photosensitive drum of SeTe or As.sub.2 Se.sub.3 for a photo receptor, and positively charged toner powder. In the printing unit, positive charges are applied to a photo receptor 23a by means of a corona charger 24a, a light pattern is irradiated onto the photo receptor 23a in an exposure unit 25a including a laser optical system, an LED, and the like, to thereby form a latent electrostatic image thereon. The latent image is developed into a toner image 7 by a developing unit 26a. A corona charging unit 27a applies negative charges to the reverse side of a print sheet of paper 5. The toner image 7 is transferred to the print sheet of paper 5, from the photo receptor 23a. The printing unit having the function stated above is referred to as a first printing unit. A toner polarity reversing device, which consists of a combination of a negatively charging corona charger 28 and a positively charging corona charger 29, reverses the polarity of the positively charged toner image 7. A second printing unit consisting of a photosensitive drum 23b transfers a positively charged toner image 6 on the reverse side of the sheet 5 in a similar way. As a result, the toner images 6 and 7 of different polarities are formed on both sides of the sheet 5. The toner images 6 and 7 are fused by heat radiated from a non-contact heating device 31 formed of an infrared lamp 30 and a reflecting plate, and fixed on the paper. In the figure, reference numeral 1 designates a transport roll 1, and 32 is a paper transport drive roll. The transport roll 1 is used for changing the direction of transporting a continuous paper and serves also as a buffer when the paper is transported. In this respect, this roll is an essential component part. The transport roll 1 comes in contact with an unfixed toner image. Therefore, there is the possibility that the surface of the transport roll is soiled with toner, and the toner image is disarranged. Japanese Patent Laid-Open Publication No. Hei. 7-072776 discloses a technique to solve the problem. In the publication, as shown in FIG. 7, a teflon coating 33 is formed on the surface of a metal roll 3. Charges of the same polarity (positive polarity in FIG. 7) as the charging polarity of the toner image 6 are applied to the surface of the teflon coating 33 by a corona charger 13, whereby making it hard for the toner image 6 to attach to the transport roll surface. Reference numeral 9 designates a cleaning means for cleaning the transport roll 1.
In a technique disclosed in Japanese Patent Laid-Open Publication No. Hei. 5-216363, an input roller made of conductive PFA is located at the extreme end of the transport guide, and a print substrate is transported to the fusing unit by the input roller.
In the publication No. Hei. 7-072776, as shown in FIG. 7, the teflon coating 33 is of the insulation nature. Because of this, charge from the corona charger 13 is accumulated in the teflon coating, a surface potential of the transport roll increases, and in an extreme case, the accumulated charge is discharged. "Disarrangement of the toner image and the background of the image", which are caused by the surface potential increase owing to the accumulated charges, will be described with reference to FIG. 8.
Generally, the discharging along the corona wire is not always uniform. In a location on the teflon coating 33 where charge 17 is increased in amount, the toner particles of a toner image 6' on the sheet 5 receives a strong repulsion from the transport roll surface, and are scattered. The toner particles 7' also undergoes an abrupt change in the potential distribution, so that the toner particles are scattered. It is found that these phenomena disarranges the toner image and creates a fog on the background. In the case of a line image of which the amount of toner attached thereto is larger than a solid image, an amount of its toner is large. A nonuniformity of the transport potential on the transport roll greatly affects the scattering of toner particles. To avoid this, it is essential to uniformize the surface potential on the transport roll surface. It is also found that to secure a stable paper transporting, it is necessary 1) to reduce an attaching force of toner particles to the transport roll surface and 2) to increase an attaching force of it to the paper.
In the publication No. Hei. 5-216363, a print substrate enters the fusing unit by way of a transport guide and a small input roller. At this time, toner attaches to the input roller, and the attached toner particles enter a gap between the input roller and the transport guide. The result is to impede the rotation of the input roller and sometimes soils the paper.